Preparation And Application Of Covalent Organic Framework Nanofiltration Membrane For Dye/salt Separation

In recent years,with the continuous development of computer technology and precision mechanical technology,the application of digital printing has become more and more extensive.Since the inorganic salt components in industrial dyes can cause corrosion to printing equipment,the dyes used in digital printing inks often need to be desalted.At present,nanofiltration technology is mostly used for dye desalination,while the traditional nanofiltration membrane pore size is uneven,and is often limited by the Trade-Off effect,which cannot take into account the retention rate and solvent flux of the dye.Covalent organic frameworks（COFs）have great potential in the field of nanofiltration due to their homogeneous pore size,stability,and unique modifiability.Relevant literature shows that COF nanofiltration membrane has high water flux while maintaining high dye retention due to its rich internal pore structure,so the application of COF materials to dye desalination has strong feasibility and broad research prospects.Inspired by the adjustable structure of COF materials,based on the relevant mechanism of nanofiltration membranes,two types of COF nanofiltration membranes were designed and synthesized.One is to synthesize three COF nanofiltration membranes with different pore sizes by selecting synthetic monomers of different sizes:COF_TAM/BDA,COF_TAM/BPA,COF_TAM/TPA,with pore sizes of 8.7（?）,12.9（?）and 16.8（?）,respectively.The simulated nanofiltration test of salty dyeing solution showed that the three nanofiltration membranes had a high pure water flux(102.5-139.7 L·m^-2·h^-2·bar^-1),the interception rate of the common reactive dyes selected was more than 98%,the interception rate of Na Cl and Na₂SO₄ was less than 12%and 21%,and the filtration flux of the dye solution could also be maintained above 49.0 L·m^-2·h^-2·bar^-1,which could achieve efficient desalination.It is also found that increasing the pore size of the nanofiltration membrane can effectively increase the solvent flux,thereby improving the nanofiltration efficiency.Second,this experiment also synthesized a cationic COF nanofiltration membrane by using the charging effect of the nanofiltration membrane,and the simulated salt-containing dyeing solution nanofiltration test showed that the electrostatic repulsion provided by the positive charge on the surface of the nanofiltration membrane can improve the retention rate of cationic dyes of smaller molecules,and the retention rate of anionic dyes of smaller molecules is reduced due to the action of electrostatic attraction.For anionic dyes with larger molecules,their retention performance is hardly affected,because the screening effect plays a dominant role at this time.The study also found that the charging effect acts on both the anion and the cation of the inorganic salt,and the filtration effect depends on the electrostatic attraction or electrostatic repulsion difference between the anion and the membrane surface charge,for example,the retention rate of cation COF to Na Cl（10.01%）is not significantly different from the above-mentioned non-charged COF nanofiltration membrane to Na Cl,but the retention rate of cationic COF on Na₂SO₄ can be as low as 11.26%.In this experiment,the factors affecting the nanofiltration effect were also explored.The results show that higher operating pressure and higher feed temperature can increase the nanofiltration flux,while excessive dye concentration will reduce the filtration flux due to concentration polarization,and higher inorganic salt concentration can also promote the permeation of inorganic salt to a certain extent.The COF nanofiltration membrane in this experiment is grown in situ,by amino pre-modification of the surface of alumina ceramics,and then using the reaction site to make the alumina support layer and the COF material separation layer through chemical bonding,so the constructed COF nanofiltration membrane has good stability,and can still maintain good retention performance and solvent flux after 20 repeated nanofiltration tests.